Robust spectroscoptic optical probe

ABSTRACT

Disclosed is a spectroscopic probe apparatus useful for Raman, near infrared, luminescence, ultraviolet or visible spectroscopies that is formed with a robust method of construction using a molten metal soldering technique. The disclosed method and apparatus provides an optical probe that is easy to manufacture yet able to withstand drastic environmental conditions without damage and produce useful spectroscopic results under such conditions.

This is a divisional application of copending application Ser. No.08/450,597, filed May 25, 1995.

FIELD OF THE INVENTION

This invention relates to spectroscopic optical probes and moreparticularly to an optical probe of robust construction especiallysuitable for in situ spectroscopic measurements.

BACKGROUND OF THE INVENTION

Various spectroscopic techniques are routinely used to determine theconstitution of chemical compositions and to monitor the progress ofchemical reactions and processes. The choice of technique, including thewavelength of the radiation employed, depends on the informationdesired.

Infrared (IR) spectroscopy is based on the interaction with chemicalsubstances of infrared irradiation having a wavelength between 0.77 μmand 1000 μm. A segment of IR spectroscopy, referred to as near infrared(NIR) spectroscopies, uses radiation wavelengths between 0.77 μm and 2.5μm. IR and NIR spectroscopies generally involves the absorption ofradiation as it passes through a sample. The absorption frequenciesprovide information regarding the chemical and physical characteristicsor the molecular structure of the irradiated substance.

Ultraviolet (UV) and visible (VIS) spectroscopic methods employ UVradiation having wavelengths between 10 nm and 350 nm and visibleradiation with wavelengths between 350 nm and 770 nm. UV/VIS techniquesmeasure the absorption of the exposing radiation by molecular electronictransitions; the particular wavelengths absorbed are characteristic ofthe molecular structure of the substance under investigation.

Raman spectroscopy is another means by which chemical, physical, andmolecular information of materials can be obtained. Incident radiationinteracting with a material undergoes scattering, which occurs in alldirections; the radiation may be scattered elastically or inelastically.The inelastically scattered radiation is referred to as Raman scatter.The wavelengths and intensities of this radiation comprise a Ramanspectrum that provides chemical and structural information regarding theirradiated material.

Luminescence spectroscopy involves the measurement of photon emissionfrom molecules. It includes photoluminescence such as fluorescence andphosphorescence, which are emissions from a substance resulting from itsexcitation by radiation absorption, and chemiluminescence, where theemission is induced by a chemical reaction. The emitted radiation ischaracteristic of the molecular structure.

All of these spectroscopic techniques are useful for gaining qualitativeand quantitative information about a chemical material. IR, NIR, andRaman spectra, however, provide the greatest amount of molecularstructural information.

Determining the constitution of a chemical composition or monitoring theprogress of a chemical reaction is frequently carried out with materialssituated in inhospitable environments. For example, analysis may berequired of a process stream under conditions of high temperature and/orpressure or in the presence of corrosive substances or powerfulsolvents. It is well known to place spectrophotometric apparatus such asa spectrograph and a radiation source in a location remote from asubstance that is to be analyzed in situ and connect the apparatus tothe sampling site by radiation conduits comprising optical fibers. Theinterface between these optical fibers and the process environment iscommonly provided by a probe, often referred to as a spectroscopicoptical probe or a fiber optic probe.

A variety of spectroscopic probes are known in the art. U.S. Pat. No.3,906,241, for example, describes a probe for use in analyzing fluidsthat incorporates three fiber optic channels, one to carry radiationfrom a source to the probe detecting head, a second to return radiationfrom the head, and a third to carry the scattered Raman radiation todetector means. In U.S. Pat. No. 4,573,761 is described a probe thatcomprises at least one optical fiber for transmitting light into asample and at least two optical fibers for collecting radiation from thesample, the collecting fibers converging with the axis of thetransmitting fiber at an angle less than 45 degrees. U.S. Pat. No.4,707,134 describes a probe comprising a plurality of converging opticalfibers contained in a housing that is closed at one end by a transparentwindow. A method for in situ detection of a compound by Ramanspectroscopy is disclosed in U.S. Pat. No. 4,802,761, wherein acollecting cell is connected by an optical fiber bundle to a remotesensing device.

PROBLEM TO BE SOLVED BY THE INVENTION

As just noted, spectroscopic probes have been described in the priorart, and several such devices are available from various vendors. Theseknown probes are frequently of complex design and thus expensive tomanufacture; they may include, for instance, precisely alignedarrangements of multiple optical fibers, lenses, and windows, as well asgasketing materials and adhesives for assembling and sealing the probecomponent parts. In harsh process environments, such probes aresusceptible to damage by high temperature and pressure and powerfulchemical solvents, resulting in leakage, misalignment, and other formsof deterioration that adversely affect probe performance. Thus, there isa need for readily manufacturable spectroscopic probes whose robustdesign and construction allow their use in drastic environments withoutperformance-degrading damage. This need is well met in the spectroscopicoptical probe of the present invention.

SUMMARY OF THE INVENTION

In accordance with the invention, a fluid-tight spectroscopic opticalprobe comprises a fluid-tight housing, with a tip having a terminalsurface sealably closing one end of the housing; one fiber opticexcitation channel terminating at the terminal surface of the tip andextending within the length of the housing and transmitting radiationfrom a radiation source to a chemical composition that is to beirradiated for spectroscopic analysis; and one fiber optic collectionchannel terminating at the terminal surface of the tip and extendingwithin the length of the housing and transmitting radiation from theirradiated composition to detector means remotely situated from thecomposition; wherein the fiber optic excitation and collection channelsare securely held and sealed within the tip by solder means encompassingeach channel.

A method for forming the spectroscopic optical probe of the inventioncomprises forming a cavity at the terminal surface of the tip andfilling the cavity with molten metal solder, which is allowed to cooland solidify to a plug; forming closely spaced holes of circularcross-section that penetrate through the tip and the plug, thecross-section of the holes being very slightly larger than thecross-section of the optical fibers comprising the excitation andcollection channels; inserting one end of each of the optical fibersinto the holes through the tip and into the plug, the ends of theoptical fibers forming a substantially continuous surface with theterminal surface of the tip; heating the solder plug to a temperaturesufficient to cause the solder to flow around the optical fibers;cooling the plug to ambient temperature, thereby securing and sealingthe optical fiber ends in the plug at the terminal surface of the tip;and grinding and polishing the substantially continuous terminal surfaceof the tip to form an optically smooth terminal surface.

ADVANTAGEOUS EFFECT OF THE INVENTION

The present invention provides an optical probe that is easy tomanufacture yet able to withstand drastic environmental conditionswithout damage and produce useful spectroscopic results under suchconditions.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a fragmentary isometric view, partially in section, of apreferred embodiment, depicting in detail the optical fiber ends at theprobe tip.

FIG. 2 is a fragmentary cross-section of the embodiment of FIG. 1.

FIG. 3 is an end elevational view of the embodiment of FIG. 1.

FIG. 4 is a fragmentary cross-section of an embodiment that includes areflector cap with a concave mirror.

DETAILED DESCRIPTION OF THE INVENTION

In FIGS. 1, 2, 3 is shown a preferred embodiment of the presentinvention. Optical probe 100 comprises a housing 101, closed at one endby tip 102. Two optical fibers 103 and 104, one serving as an excitationchannel for radiation from a source to a composition to be analyzed, theother comprising a collection channel for transmitting radiation fromthe irradiated composition to remotely situated detector means, extendfrom the terminal surface 105 of the tip through the length of thehousing. The depicted embodiment includes a single excitation channeland a single collection channel, which simplifies its construction. Astaught in U.S. Pat. No. 4,707,134, the disclosures of which areincorporated herein by reference, a single fiber is sufficient totransmit light from a light emitting diode, a laser, or a diode laser.However, a probe of the invention may contain two to six fiber opticcollection and/or excitation channels. In one useful configuration ofthe fibers, a single excitation channel within a bundle is surrounded bysix collection channels, as shown in U.S. Pat. No. 4,573,761, thedisclosures of which are incorporated herein by reference.

On the terminal surface 105 of tip 102 is formed a cavity 106, which isfilled with molten metal solder that solidifies on cooling to a solderplug 107. This plug comprises means for securing and sealing the ends ofthe optical fibers at the terminal surface of the probe tip.

In the construction of the probe of the invention, closely spaced holesof circular cross-section very slightly larger than the fibercross-section are drilled through the tip and the solder plug. Anoptical fiber is inserted in each hole to fill it, the end of each fiberforming a substantially continuous surface with the terminal surface ofthe tip. A cavity 108 may be formed in the back of the tip to facilitateinsertion of the fibers into the holes.

The solder plug is heated to a temperature sufficient to cause thesolder to flow around the optical fiber ends. During heating; a smalladditional quantity of solder may be applied to the plug to compensatefor settling. On cooling of the plug to ambient temperature, the fibersare sealed and secured at the terminal surface of the tip. Subsequentgrinding and polishing of the tip terminal surface provides an opticallysmooth surface.

In sealing and securing the optical ends at the terminal surface of thetip, the heating of the solder plug must be carefully controlled toavoid softening the quartz glass of the fiber. Damage to the fiberswould be highly likely should one attempt to secure them by drillingholes large enough to accommodate them through the tip (which ispreferably made of stainless steel), inserting the fiber ends in theholes, and then applying solder to effect sealing. In accordance withthe method of the invention, the likelihood of damage to the fibers isminimized by the formation of the solder plug at the tip terminalsurface prior to the drilling of the holes to receive the fibers. Theholes should be of a diameter just large enough to allow the fibers tobe inserted into them; the solder plug is then heated to a point justsufficient to cause the solder to flow around the fiber ends, so that oncooling the ends are secured and sealed in the tip. To further reducethe amount of heat required for sealing and also to avoid unevenexpansion within the probe tip during use, it is desirable that the sizeof the solder plug be of the minimum size necessary to ensure securesealing of the fibers.

The optical fibers utilized in the probe of the invention are preferablystep-indexed multimode fibers, which are available from severalcommercial sources, for example, Fiberguide, Stirling, N.J. Theirdiameters may range from 1 μm to 1000 μm, preferably 200 μm to 400 μm.For Raman and luminescence spectroscopy fibers of about 200 μm diameterare preferred; for UV/VIS and NIR measurements, fibers with diameters ofabout 400 μm are preferred.

Preferred optical fibers for the present invention comprise a quartzglass core 109 surrounded by a thin inner layer of doped quartz 110, andthen by a thin solder-adherable metallic outer layer 111, preferably ofgold. The metallic layer 111 facilitates adhesion of the fiber to thesolder plug, as described in U.S. Pat. No. 4,033,668, the disclosures ofwhich are incorporated herein by reference.

The housing and tip of the probe can be constructed from variousmetallic materials, for example, copper or titanium. Preferred metalsfor this purpose include Hastalloy and stainless steel. The solderemployed to seal and secure the optical fiber ends in the tip should beresistant to chemical deterioration and have a fusion temperature highenough to withstand the drastic conditions to which the probe may besubjected during use. A presently preferred embodiment can be employedwithout damage at temperatures of about 500° C. and pressures ranging upto 30,000 psi. Of course, the solder fusion temperature should also besufficiently low to achieve sealing of the optical fibers withoutsoftening of the glass. Preferred soldering material for securing theoptical fiber ends, with fusion temperatures in the range of about 600°C. to 1100° C., are silver or gold solders; other types of solders maybe employed to connect the probe tip to the housing.

In constructing the probe, a small amount of additional length, up toabout 0.5 percent of the total, is provided in the optical fibersextending through the housing away from the tip. This slack is desirableto prevent stresses and cracking of the fibers as the metal componentsof the probe expand at different rates than the optical glass at hightemperatures. The ends of the fibers remote from the tip are connectedwith the radiation source and spectrograph by connecting means wellknown in the art, for example, SMA connectors.

The detector means, which may be an ultraviolet-visible (UV/VIS), a nearinfrared (NIR), a luminescence, or a Raman spectrometer, and theradiation source may be situated at a distance from and connected to theprobe by optical fibers, as described in U.S. Pat. No. 4,802,761, thedisclosures of which are incorporated herein by reference. The type ofradiation source depends on the particular spectrometry; useful sourcesinclude, for example, argon, hydrogen, deuterium, xenon, and tungstenlamps for UV/VIS; nichrome wires, Nernst glowers, and halogen-modifiedtungsten lamps for NIR/IR; and lasers, especially diode lasers, forRaman and luminescence spectrometry.

Following construction, the probe may be inserted into a line or areactor within a process and secured therein in a fluid-tight fashion bymeans such as, for example, a threaded, soldered, or otherwise sealableconnection. Although the robust construction of the probe of theinvention makes it especially useful for monitoring chemicalcompositions in harsh environments characterized by high temperaturesand pressures, it is not restricted to such applications. A probe may beconstructed, in accordance with the method of the invention, with shapeand dimensions suitable for in situ analyses, Raman spectrometricmeasurements for example, in living organisms, as described in U.S. Pat.No. 3,906,241, the disclosures of which are incorporated herein byreference.

Fiber optic probes such as those of the present invention areparticularly useful for Raman spectrometric measurements, as describedin Schwab et al., Anal. Chem., 1984, vol. 56, pages 2199-2204, thedisclosures of which are incorporated herein by reference. A probe ofthe invention may also be utilized for transmittive/reflectivespectroscopy such as UV/VIS and NIR measurements. FIG. 4 depicts theembodiment of FIG. 1 further comprising a reflector cap 201 thatincludes a concave mirror 202 and is provided with flow-through ports203. The ports enable the chemical composition to flow over the probetip, and the mirror is constructed with appropriate curvature and ispositioned at the proper distance at the tip to maximize the amount ofradiation from the irradiated composition that is gathered by thecollection channel.

The present invention provides an optical probe of simple design andready manufacturability whose robust construction enables its prolongeduse for obtaining reliable spectrometric measurements, even in harshenvironments of elevated temperature and pressure. The excellentperformance of the probe of this invention under such conditionscontrasts with that of several probes that are commercially availablefrom various vendors and are advertised as able to withstand elevatedpressures as well as temperatures as high as 300° C. In a polymerproduction process at temperatures of 200-300° C., under whichconditions a probe of the invention yielded reliable measurements over aprolonged period, one commercially available probe that included opticalrods, lenses, and windows cemented into a metal housing quickly failedas a result of the rods becoming loose and misaligned. In anothercommercial probe of a different design, the epoxy resin employed to holdthe fibers of a bundle in place carbonized and decomposed during a brieftrial under the process conditions. In still another instance, theprotective window seal of the probe failed, resulting in leakage of thepolymer into the chamber containing the optical fibers. Finally, duringtesting of another commercial probe touted as able to withstand 300° C.and high pressure, its protective sapphire window was lost to theproduction stream and never recovered. These repeated failures of avariety of commercially available optical probes after limited exposureto harsh process environments attest to the remarkable and unexpectedadvantages provided by the robust probe of the present invention.

The following examples further illustrate the invention.

EXAMPLE 1 Raman Probe for Monitoring a Batch Chemical Reaction

A fiber optic probe was constructed as shown in FIGS. 1-3. Two 400-μminner diameter gold coated fiber optics purchased from Fiberguide,Stirling N.J., were silver soldered using Safety Silv® 45 solderobtained from J. W. Harris Co., Cincinnati, Ohio, into a 0.25 inchdiameter 316 stainless steel tube body. Upon cooling, the probe tip waspolished to a mirror finish. A test of the probe for signal throughputconfirmed its efficiency. The probe was used to monitor a chemicalreaction by Raman spectrometry over a period of three hours, duringwhich the temperature reached 220° C. Excellent spectral data werecollected, and the probe showed no signs of degradation.

EXAMPLE 2 Raman Probe for Monitoring a Manufacturing Process

A fiber optic probe was constructed similar as shown in FIGS. 1-3. Two200-μm inner diameter gold coated fiber optics purchased fromFiberguide, Stirling N.J., were silver soldered into a 0.25 inchdiameter 316 stainless steel tube. Upon cooling, the probe tip waspolished to a mirror finish and subsequently tested for signalthroughput. The probe was placed in a process stream of a manufacturinginstallation and connected to a Raman analytical instrument. The processwas operated at pressures between 15 to 30 psi and temperatures between200° C. and 230° C. After several months of successful spectralacquisition, the probe was removed from the process. Inspection of theprobe following removal showed that it had robustly survived the lengthyexposure to the process environment.

EXAMPLE 3 NIR Probe for Monitoring a Manufacturing Process

A fiber optic probe for NIR spectroscopy was designed and constructedwith a reflector cap containing a concave mirror, as shown in FIG. 4.Two 400-μm inner diameter gold coated fiber optics were silver solderedinto a 0.25 inch diameter 316 stainless steel tube. Upon cooling, theprobe tip was polished to a smooth, mirror-like finish. A reflectorcomprising a concave mirror was fastened above the probe tip to completethe probe assembly. This probe was placed in a manufacturing processstream operating at temperatures up to 300° C. and pressures up to 1000psi. After two months of spectroscopic measurements, the probe wasremoved from the process. Inspection of the probe after its removalconfirmed that it had withstood the prolonged exposure to the harshconditions of the process.

EXAMPLE 4 Probe for UV/VIS Monitoring of Color in an Extruder

The fiber optic probe of Example 3 was inserted into an extruder andused to monitor the UV/VIS spectrum of a molten polymer. The operatingconditions of the stream were temperatures up to 315° C. and pressuresup to 250 psi. The probe performed well over several days of testing.

The invention has been described in detail with particular reference topreferred embodiments thereof, but it will be understood that variationsand modifications can be effected within the spirit and scope of theinvention.

What is claimed is:
 1. A method for forming a fluid-tight spectroscopicoptical probe that comprises a fluid-tight housing and a tip having aterminal surface and sealably closing one end of said housing, saidhousing and tip being formed of metal; an excitation channel thattransmits radiation from a radiation source to a chemical composition tobe irradiated and a collection channel that transmits radiation from anirradiated chemical composition to detector means, each said channelcomprising an optical fiber having a circular cross-section, one end ofeach said fiber terminating at said terminal surface of said tip andextending within the length of said housing; said method comprising:(a)forming a cavity at said terminal surface of said tip; (b) filling saidcavity with molten metal solder, then allowing said solder to cool andsolidify to a plug; (c) forming closely spaced holes of circularcross-section penetrating through said tip and said plug, thecross-section of said holes being very slightly larger than thecross-section of said optical fibers; (d) inserting one end of each ofsaid optical fibers into one of said holes through said tip and intosaid plug, thereby filling said holes in said tip and plug, said endsforming a substantially continuous surface with said terminal surface ofsaid tip; (e) heating said solder plug to a temperature sufficient tocause the solder to flow around said optical fiber ends; (f) coolingsaid plug to ambient temperature, thereby securing and sealing saidoptical fiber ends at said terminal surface of said tip; and (g)grinding and polishing said substantially continuous terminal surface ofsaid tip, thereby forming an optically smooth terminal surface.
 2. Themethod of claim 1 wherein said metal solder comprises a silver solder ora gold solder.
 3. The method of claim 1 further comprising sealablyconnecting said housing with said tip by soldering.
 4. The method ofclaim 1 wherein said housing and tip are formed of Hastalloy orstainless steel.
 5. The method of claim 1 wherein each said opticalfiber further comprises a peripheral surface bearing a thinsolder-adherable layer of gold.
 6. The method of claim 1 wherein eachsaid optical fiber has a diameter of 200 μm to 400 μm.
 7. The method ofclaim 1 wherein each said optical fiber comprises a step-indexedmultimode optical fiber.